Silver-aluminum mixed oxide catalyst (Ag-Al2O3) prepared by the sol-gel method was studied for the selective reduction of NO by various alkanes in the presence of water vapor. As the carbon number of alkanes increases, the de-NOx activity and water tolerance were markedly increased. In the case of n-octane as a reductant, the presence of water vapor markedly promoted NO reduction. The results of reaction studies and in situ IR experiment showed that the possible reasons for the promoting effect by water vapor are the inhibition of the n-octane oxidation by O-2 and the suppression of the poisoning effect caused by carboxylate and carbonate species. Among various alumina-supported transition metal catalysts, Ag-Al2O3 showed the highest activity for SCR by n-octane. Ag-Al2O3 showed higher NO conversion to N-2 and selectivity than alumina-supported Pt and Cu-ZSM-5 catalysts for the selective reduction of NO by n-octane and i-octane. (C) 2000 Elsevier Science B.V. All rights reserved.